The present invention relates to a method for balancing an energy storage system, comprising a plurality of connected battery cells, in a vehicle comprising an electric machine arranged for propulsion of said vehicle, said method comprising: monitoring a need for balancing of said cells; initiating a request for said balancing based on the operational condition related to said battery cells; discharging said energy storage system until the state of charge of said energy storage system has reached a predetermined level; and balancing the voltage of said battery cells.
The invention also relates to an arrangement for balancing an energy storage system, comprising a plurality of connected battery cells, in a vehicle comprising an electric machine arranged for propulsion of said vehicle, said arrangement comprising a battery management unit which is configured for: monitoring a need for balancing of said cells; for initiating a request for said balancing based on the operational condition related to said battery cells; for discharging said energy storage system until the state of charge of said energy storage system has reached a predetermined level; and for balancing the voltage of said battery cells.
In the field of vehicles, there is a steady increase in research and development related to propulsion of vehicles with alternative power sources, i.e. power sources being used as alternatives to conventional internal combustion engines. An internal combustion engine, for example in the form of a gasoline engine or a diesel engine, offers high efficiency with relatively low fuel consumption. However, environmental concerns have led to an increase in development of more environmental-friendly power sources for vehicles. In particular, the development of electrically operated vehicles has emerged as a promising alternative.
Today, there exist various types of vehicle propulsion systems comprising electric machines. For example, a vehicle can be operated by means of an electric machine solely, or by means of an arrangement comprising both an electric machine and an internal combustion engine. The latter alternative is often referred to as a hybrid vehicle (HEV), and can for example be utilized in a manner in which an internal combustion engine is used for operating the vehicle while driving outside urban areas whereas the electric machine can be used in urban areas or in environments in which there is a need to limit the discharge of harmful pollutants such as carbon monoxide and oxides of nitrogen.
The technology involved in electrically operated vehicles is closely related to the development of electrical energy storage systems, for example in the form of battery-related technology for vehicles. Today's electrical energy storage systems for vehicles may comprise a set of rechargeable battery cells which, together with control circuits, forms a unit which is arranged in a vehicle and which is configured for operating an electric machine. A hybrid vehicle is also often arranged so that the energy storage system is charged during braking, by means of a process known as regenerative braking.
A vehicle being operated by means of an internal combustion engine and an electric machine supplied with power from a rechargeable electrical energy storage system is sometimes referred to as a plug-in hybrid electric vehicle (PHEV). A plug-in hybrid electric vehicle uses an energy storage system with rechargeable batteries or another suitable energy source which can be restored into a condition involving a full charge through a connection to an external electric power supply.
The external power supply can be in the form of the common electric grid power system which can be accessed via a conventional power cord, or can be in the form of other arrangements depending on the vehicles involved and the power need for the recharging process. In case of vehicles in the form of buses or heavy transport vehicles, more powerful charging devices and procedures are normally needed as compared with smaller cars and similar vehicles.
The electric energy storage systems in electric and hybrid electric vehicles commonly comprise battery packs of multiple series-connected electrochemical cells, typically hundreds of lithium cells. During operation of the vehicle, the battery pack is gradually discharged while it supplies energy to the electric machine of the vehicle. However, no two cells are identical—there are always slight differences in for example capacity, state of charge and self-discharge rate of the cells. The capacity and lifetime of the weakest cell limit the capacity and lifetime of the battery pack as a whole. In order to be able to obtain more energy and greater lifetime from the battery pack, the voltage is periodically redistributed among the cells in order to bring them all to a common state of charge. This is commonly referred to as cell balancing. One known method for cell balancing is to discharge individual cells of the battery pack over balancing resistors inside the battery.
An energy storage system of the above-mentioned type is associated with a battery management unit which is configured for monitoring the batter cells and for monitoring any need for balancing of the battery cells. A condition for initiating a cell balancing procedure is suitably that there is a difference in the so-called open cell voltage between any two battery cells which exceeds a predetermined value, for example 0 mV. Such a condition can be regarded as suitable for the battery management unit to start balancing the cells.
It is previously known that cell balancing is preferably performed at a state of charge level wherein the derivative of the energy storage system output voltage with respect to the energy storage system state of charge dOCV/dSOC is relatively high. This means that detection of the open cell voltage of the battery cells—which is used for determining whether cell balancing is necessary—can be made with a high degree of accuracy.
This also means that the actual start of a cell balancing procedure may have to be postponed until a point in time at which the state of charge is of a magnitude in which said derivative is sufficiently high. This may result in a very long waiting time for providing this cell balancing, since the energy storage system may have to be partly discharged before the cell balancing can be carried out. In summary, cell balancing according to prior art is a process which takes a long time to carry out.
Also, cell balancing is normally not carried out before the battery pack has had a so-called relaxation time, i.e. a certain time to rest. This also contributes to the long time period until the cell balancing can be finished.
In order to limit a temperature increase in the battery pack, the discharge power has to be low which in turn also contributes to an overall long battery cell balancing time.
The patent document JP 2010-081731 discloses a cell balancing system in which a navigation system and information about the traveled route can be used for controlling the cell balancing procedure.
Even though the system according to said JP 2010-081731 may contribute to a solution in which the time for cell balancing may be reduced, there is still a need for an improved battery balancing method removing the above mentioned disadvantage related to a long cell balancing time.
It is desirable to provide a method for balancing an energy storage system, in particular for a hybrid electric vehicle, in which the above-mentioned problem is at least partly eliminated.
The disclosure concerns, according to an aspect thereof, a method for balancing an energy storage system, comprising a plurality of connected battery cells, in a vehicle comprising an electric machine arranged for propulsion of said vehicle. The method comprises monitoring a need for balancing of said cells; initiating a request for said balancing based on the operational condition related to said battery cells; discharging or charging said energy storage system until the state of charge of said energy storage system has reached a predetermined level; and balancing the voltage of said battery cells.
Furthermore, the method comprises allowing input instructions representing a time schedule for said balancing upon initiating said request for balancing; and initiating said discharging so that the energy storage system reaches said predetermined level of the state of charge in accordance with said input instructions.
An advantage of an aspect of this disclosure is obtained through the fact that may be carried out at considerably less time that according to prior art, if for example said input instructions are provided by a driver of said vehicle in order to schedule the balancing at a suitable point in time, for example in connection with a nightly stop of the vehicle or even during a short lunch break stop. In such case, the balancing can be carried out at a point in which the energy storage system has reached a suitable state of charge for allowing an optimal cell balancing procedure.
Further advantages are achieved by implementing one or several of the features of the dependent claims.
According to an embodiment, the method comprises initiating the request for balancing based on values representing the state of charge and/or the open cell voltage of said battery cells. Suitably, the method comprises initiating said request for balancing in the event that the difference of the open cell voltage between any two battery cells exceeds a predetermined threshold value. This means that the cell balancing is carried out when certain suitable conditions are fulfilled.
Furthermore, according to an embodiment, the input instructions which are allowed as mentioned above can be in the form of manual input from a driver of said vehicle. This manual input can correspond for example to information related to a point in time at which the driver wishes to initiate the cell balancing. This could be a point in time which is suitable having regard to the operation of the vehicle, i.e. it could be a point in time which corresponds to a lunch break or a similar stop. According to a further embodiment, which also can be combined with the above-mentioned embodiment, the input instructions can be in the form of data transmitted from a remote control unit. Such a remote control unit can for example be constituted by a company which is an owner of the vehicle in question (for example in the form of a bus) and which consequently may determine a suitable point in time for initiating the cell balancing. Such a point in time may be related to a scheme of operation of the vehicle so that the cell balancing can be carried out during a standstill of the vehicle.
According to an embodiment, the method may also comprise a step of transmitting data related to the need for balancing of said battery cells from said vehicle to said remote control unit.
Furthermore, the method preferably comprises discharging or charging said energy storage system when the state of charge of said energy storage system is within an interval when the derivative of the present energy storage system output voltage with respect to the present energy storage system state of charge dOCV/dSOC is more than two times higher than a minimum derivative of the energy storage system output voltage with respect to the energy storage system state of charge.
Furthermore, the method may comprise relaxation (i.e. a time to rest) of said energy storage system before said balancing.
The object of the present disclosure can also be obtained by means of an arrangement for balancing an energy storage system, said arrangement comprising a plurality of connected battery cells, in a vehicle comprising an electric machine arranged for propulsion of said vehicle. The arrangement comprises a battery management unit which is configured for: monitoring a need for balancing of said cells; for initiating a request for said balancing based on the operational condition related to said battery cells; for discharging or charging said energy storage system until the state of charge of said energy storage system has reached a predetermined level; and for balancing the voltage of said battery cells. The arrangement is furthermore arranged so that said battery management unit is further configured for allowing input instructions representing a time schedule for said balancing upon initiating said request for balancing; and for initiating said discharging so that the energy storage system reaches said predetermined level of the state of charge in accordance with said input instructions.